Secure communication alarm system

ABSTRACT

A secure fiber optic communication that is based on physical properties of light propagating through a Sagnac interferometer in combination with optical power alarms. These alarms are then used to alert users to intrusion and to shut down communications to avoid compromising data.

BACKGROUND OF THE INVENTION

This invention relates generally to fiber optic communication systemsand, more particularly, to Sagnac interferometer based fiber opticsystems that utilize counterpropagating light paths to form a fiberoptic communications loop. Alarm systems, which are adapted to detectchanges in the optical characteristics in the loop, including changes inthe optical power or optical frequency, are used to avoid the risk ofcompromising data and to alert users to an intrusion attempt.

The need for high bandwidth secure communication systems that areamenable to use in networks and which minimize the need for encryptionis becoming increasingly acute. Long-term trends for very high bandwidthsensors, computers, and multichannel video display capabilities willdictate the specific designs utilized in serving this type oftechnology. The advent of fiber optics has opened up a new era wherevery high speed, low cost telecommunication is possible.

SUMMARY OF THE INVENTION

There is provided by this invention a Sagnac interferometer based securecommunication system using a short coherence length light source incombination with counterpropagating paths that have inherentself-matching characteristics to make an intrusion resistanttelecommunications link.

In one of the simplest forms of the Sagnac interferometer based securecommunication system, light from a broadband light source such as alight emitting diode is directed into a first port of a beamsplitter. Asecond port of the beamsplitter is directed toward a detector whichmonitors amplitude fluctuations of the light source. Usually the lightbeam is directed through polarizing and spatial filter elements beforeintroduction into the beamsplitter to ensure that counterpropagatingbeams through the interferometer loop transverse the same path. Thethird and fourth ports of the beamsplitter enable a split light beam tocounterpropagate about a common path. Data is impressed upon thecounterpropagating light beams by modulating their relative phase.

When the two beams of light recombine on the beamsplitter, theirdifference in phase causes the light beam that returns via thepolarizing and spatial filter elements to be amplitude modulated. Whenthe combined beams return to the beamsplitter, a portion of thisamplitude modulated light beam is directed into a detector whichconverts the light signal into an amplitude modulated electrical signalcorresponding to the data impressed onto the light beam.

The light source, beamsplitters, beam conditioning optics, detector andassociated support electronics constitute the receiver for the system.The optical and electrical support elements used to impress the relativephase difference between the counterpropagating light beams is thetransmitter for the system.

The security of the system is based on a combination of the informationbeing impressed on the relative phase difference between thecounterpropagating light beams and the low coherence length of the lightsource.

The lowest noise, highest performance Sagnac interferometers employbroadband light sources with very low coherence lengths to reduce boththe amplitude noise of the light source, and spurious noise due tocoherent backscatter throughout the fiber loop. Since informationimpressed upon the system depends upon the phase relationship betweenthe counterpropagating light beams, the two beams must be mixed toextract the signal. Since recorders do not exist at the frequencies of10¹⁴ HZ typical of light beams, this must be done in real time. Anintruder trying to tap the system would first have to access bothcounterpropagating beams if the system were to be passively tapped.

In addition, the intruder would have to access both counterpropagatingbeams and match the pathlength of the two beams to within a fewcoherence lengths characteristic of the light source in order that theamplitude modulated output signal may be constructed. Since thecounterpropagating pathlengths may differ by kilometers at the point ofinterception, and since the coherence length of a low coherence lightsource such as light emitting or superradiant diode may be on the orderof 30 microns, tapping into the system becomes an extremely difficultand time consuming task analogous to finding a needle in a haystack.

To make this system even more extremely secure a random pathlengthgenerator may be used to randomly vary the relative pathlength of thetwo counterpropagating beams. This is equivalent to having the needle inthe haystack moved randomly throughout the haystack. In the unlikelyevent the intruder manages to achieve the pathlength matching condition,a new equally difficult pathlength condition chosen totally at randomoccurs a short time later. The situation is analogous to luckilystumbling on the needle in the haystack only to have it hidden onceagain at some random location in the haystack an instant later.

An alternative approach is to utilize optical power alarm systems thatpreclude the potential intruder from obtaining information even ifpathlength matching conditions should occur. This approach is analgousto making the needle in the haystack dim and disappear.

This invention will provide a secure single mode optical fibercommunication link having very high bandwidths, such that longrepeaterless links are possible. This invention will enable utilizationof low cost single mode fiber telecommunication technology. Theinvention will also provide a system which is amenable to uses bymultiple users and networking arrangements.

The configuration of the system of the present invention will enable thesending of information at high data rates, using frequency shifters,phase modulators, tap-resistant single mode fibers, random opticalpathlength generators, and random amplitude modulation of the systemlight sources, to prevent unauthorized intrusions. Unauthorizedintrusions onto the system, through the use of appropriately configuredoptical alarm systems, will be precluded. The present invention providesflexible secure communication systems which offer adequate security atthe lowest possible cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematical representation of a fiber optic communicationsystem based on the Sagnac interferometer that incorporates theprinciples of this invention;

FIG. 2 illustrates a fiber optic communication system for multipletransmitting stations in the communications loop;

FIG. 3 illustrates a timing diagram for the phase information impressedon the counterpropagating beams in the communications loop of FIG. 2;

FIG. 4 illustrates the relative phase changes of information impressedon the counterpropagating beams by using a sawtooth or serrodynewaveform as a driver;

FIG. 5 illustrates the permissible sequence sawtooth waveforms that maybe used as drivers for the system of FIG. 4, and the effect of animpermissible sequence;

FIG. 6 shows a timing diagram for impressing a data stream onto thesystem using a series of sawtooth driving waveforms;

FIG. 7 illustrates an electronics block diagram of circuitry that couldbe used to support the impression of data onto the system using asawtooth waveform method;

FIG. 8 illustrates a simple intrusion sceanario that requires theintruder to perform pathlength matching;

FIG. 9 illustrates a fiber optic communications system with a randompathlength generator;

FIG. 10 illustrates a fiber optic communications system with a balancedoptical power alarm system;

FIG. 11 illustrates a secure fiber optic communication system with amodified balanced alarm system and a wavelength passband filter;

FIG. 12 illustrates a secure fiber optic communications system with amultispectral balanced optical power alarm system in series with thetransmitter phase modulator;

FIG. 13 illustrates a secure fiber optic communication system with amultispectral balanced optical power alarm system in parallel to thetransmitter phase modulator;

FIG. 14 illustrates a secure fiber optic communications system utilizingmultispectral light sources in combination with a multispectral alarmsystem at the transmitter, and wavelength division multiplexed opticalpaths to isolate the transmitting phase modulator;

FIG. 15 illustrates a system similar to that shown in FIG. 14, with themultispectral alarm system located with the receiver;

FIG. 16 illustrates a secure fiber optic communications system utilizinga single broadband light source in combination with a ratioedmultispectral alarm system located at the receiver;

FIG. 17 illustrates a secure fiber optic communications system similarto that of FIG. 16 but with the ratioed multispectral alarm systemlocated at the transmitter;

FIG. 18 illustrates a multispectral alarm system with a dispersiveelements in combination with detector arrays in a secure fiber opticcommunication system;

FIG. 19 illustrates a secure fiber optic communications system that is afull duplex system with dual beamsplitter multispectral alarm systems;

FIG. 20 illustrates a secure fiber optic communication system whichsupports full duplex operation with a beamsplitter multispectralintensity alarm system;

FIG. 21 shows a block diagram of the electronics used to support thetransmitter alarm system of FIG. 19;

FIG. 22 illustrates a block diagram of the electronics used to supportthe receiver alarm system of FIG. 20;

FIG. 23 is a schematic block diagram of a full duplex secure fiber opticcommunication system that uses ratioed multispectral alarm systems;

FIG. 24 illustrates in block diagram form the electronics used tosupport the alarm system for FIG. 23;

FIG. 25 illustrates a secure fiber optic communication system with aninterferometric alarm;

FIG. 26 illustrates an all optical repeater that is used to extend therange of the secure fiber optic communication system;

FIG. 27 illustrates an all optical repeater that is used to amplify thesignal level in both legs of the Sagnac loop to extend the range of thesecure fiber optic communication system;

FIG. 28 illustrates an all optical fiber repeater for the secure fiberoptic communication system based on the usage of dual core opticalfiber;

FIG. 29 illustrates an alarm system for an all optical repeater thatuses a side pumped amplifying section of fiber;

FIG. 30 illustrates an alarm system for an all optical repeater thatuses an end pumped amplifying section of fiber;

FIG. 31 illustrates an alarm system for an all optical repeater thatuses a semiconductor amplifying section.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

The Sagnac interferometer based secure communication system is basedupon the usage of a short coherence length light source in combinationwith the self matching characteristics of the Sagnac interferometer tomake an intrusion resistant telecommunication link. Since the system isbased upon the use of single-mode fiber optic cable it is compatiblewith many installed telecommunication links.

Referring to FIG. 1, there is as shown a basic Sagnac interferometerbased secure communication link 101. The system consists of a receivermodule 103 and a transmitter module 105 interconnected by the fiberoptic lines 107 and 109. The receiver module consists of a light source111, a depolarizing element 113 to enable the usage of conventional lowcost single mode optical fiber, a central fiber optic beamsplitter 115and an output detector 117. Polarization preserving fiber could be usedbut this would greatly increase cost. The transmitter module 105consists of a phase modulator 119 that is offset from the center 121 ofthe loop by the fiber length 123.

The operation of the secure fiber optic communication link of FIG. 1 isas follows. Light from the light source 111 is coupled into an opticalfiber and directed through the depolarizing element 113, which may be afiber Lyot depolarizer, to insure that light entering the centralbeamsplitter 115 is depolarized. The central beamsplitter 115 splits thelight beam into two counterpropagating beams of light which propagate indirections 123 and 125.

The light beam propagating in direction 123 then circulates through thefiber 107 and the center 121, with respect to central beamsplitter 115,and on to the phase modulator 119 which is used to impress data in theform of phase information. The light beam propagating in direction 123then passes through the fiber 109 back to the central fiber beamsplitter 115. The light beam propagating in direction 125 circulatesthrough the system in the opposite direction.

The action of the phase modulator 119 is again to impress phaseinformation upon the light beam propagating in direction 125. When thetwo light beams 123 and 125 recombine on the central fiber opticbeamsplitter 115 they will interfere with one another. This interferenceresults in an amplitude modulated light signal being directed to theoutput detector 117. The net result occurring in the system is theimpression, by the phase modulator 119, of an input data stream onto thesystem by generation of a relative phase difference between thecounterpropagating light beams which travel in directions 123 and 125.

When the two beams recombine on the central beamsplitter 115, the datawhich is carried to the receiver 103 as quickly as the phase differenceis converted to an amplitude modulated signal. The modulated signal isdetected by the output detector 117.

FIG. 2 is a diagram of the system configuration which enables theillustration of one method of impressing relative phase differencesbetween the counterpropagating light beams referred to in the discussionfor FIG. 1 above. A receiver 203 is illustrated, and is equivalent toreceiver 103 of FIG 1. Fiber optic lines 207 and 209 extend from theboundary of receiver 203. The receiver module 203 consists of a lightsource 211, a depolarizing element 213, a central fiber opticbeamsplitter 215 and an output detector 217.

A pair of transmitting stations 221 and 223 are located along a commonoptic path facilitated by fiber optic lines 207 and 209. A signal may betransmitted from either of the transmitting stations 221 or 223 to thereceiver 203. Information may be impressed by the transmitting stations221 and 223 by using a phase modulator (not shown but similar to phasemodulator 119 of FIG. 1) which operates at high data rates over acharacteristic period that depends on its position along the lightpathloop formed by fiber optic lines 207 and 209.

In particular, the transmitting station 221 is located a distance(L-L₁)/2 in a counterclockwise direction from the beamsplitter 215 whereL is the length of the complete round trip light path from beamsplitter215, along fiberoptic line 207, past transmitting stations 221 and 223,and along fiber optic line 209 back to beamsplitter 209. L₁ is thedistance between the transmitting station 221 and the symmetricallyplaced position 225. Symmetrically placed position 225 is also adistance (L-L₁)/2 from the beamsplitter 215. L₂ is the distance betweenthe transmitting station 223 and the symmetrically placed position 227.Symmetrically placed position 227 is also a distance (L-L₂)/2 from thebeamsplitter 215.

One means to impress phase information onto the counterpropagating lightbeams is to operate the phase modulator of the transmitting station 221over the time interval [0, L₁ n/c] at high data rates, wait during thetime interval [L₁ n/c, 2L₁ n/c], transmit during [2L₁ n/c, 3L₁ n/c],wait during [3L₁ n/c, 4L₁ n/c] and so forth. Here n is the index ofrefraction of the fiber optic lines 207 and 209 and c is the speed oflight in a vacuum.

A timing diagram illustrating the phase information impressed upon theclockwise and counterclockwise beams and the resulting amplitudemodulation due to their relative phase difference when the two beamsrecombine on the detector is shown in FIG. 3. The diagram corresponds tothe situation for transmitter 221 of FIG. 2.

The sequence starts at time zero when a stream of five bits ofinformation are impressed over the time interval [0, L₁ n/c] by shiftingthe phase of the counterclockwise light beam which originates at thelocation of transmitter 221 of FIG. 2. At the same time, the clockwiselight beam originating from the location 225 propagates through thefiber length L₁, and arrives at transmitter 221.

Over the next time interval [L₁ n/c, 2L₁ n/c] the phase modulator oftransmitter 221 (not shown in FIG. 2) is turned off such that thecounterpropagating light beams are not phase modulated. Since the twocounterpropagating light beams which enter the system through thebeamsplitter 215 arrive at the location 225 and the phase modulator oftransmitter 221 simultaneously, this means that the clockwise light beamwhich will mix with the counterclockwise light beam modulated over thetime interval [0, L_(l) n/c] passes through the transmitter 221 over thetime interval [L₁ n/c, 2L₁ n/c].

For ease of comparison, the relative phase modulation of the clockwisebeam is moved back in time by an interval L₁ n/c on the timing diagramof FIG. 3 and thus the interval [0, L₁ n/c] corresponds to [L₁ n/c, 2L₁n/c] for the clockwise beam, and so forth.

When the two beams are recombined at the beamsplitter 215 afterpropagating through the fiber optic path represented by L, the two beamshave a relative phase difference which is the difference between theclockwise and counterclockwise timing diagram lines shown near thebottom of FIG. 3. Notice that the information packet is repeated twicewhen the two beams are recombined, but that the sign of the relativephase has changed.

The format method just described in association with FIGS. 2 and 3relies upon bursting the data with about a 50% duty cycle to avoid"collisions" of the data. An alternative method is to format the inputto the phase modulator such that a relative phase difference, betweenthe counterpropagating light beams is established in a continuous mannerallowing the transmission of data without interruption.

FIG. 4 illustrates a basic system employing this technique. The receiver401 and transmitter 403 are connected by the fiber link 405. Fiber link405 is equivalent to fiber optic lines 107 and 109 of FIG 1. Thereceiver consists of a light source 407, depolarizing element 409,central fiber optic beamsplitter 411, output detector 413, phasemodulator 415 and bias driver 417. The transmitter 403 consists of thephase modulator 419 and offset fiber 421 arranged to ensure the phasemodulator 419 does not lie in the center of the Sagnac loop.

When data in sawtooth form 423 is fed into the phase modulator 419, theramped part of the sawtooth causes a time varying phase modulationresulting in a relative phase difference between the counterpropagatingbeams of light travelling in directions 425 and 427 as they circulatethrough the system. This is due to the offset in time of arrival of theclockwise and counterclockwise beams which is in turn due to thepresence of the offset fiber 421.

Because the ramped part of the slope is at a fixed inclination, thephase offset between the two light beams 425 and 427 is fixed. When thephase modulation of the input 423 has zero slope the net difference inphase between the two counterpropagating beams of light is zero. Thuswhen the two beams 425 and 427 circulate through the system andrecombine on the central fiber optic beamsplitter 411 an amplitudemodulated light signal will be recreated that is directed toward theoutput detector 413 to form the output digital signal 431. In generalthe input to the secure fiber optic communication system at the phasemodulator 419 will be differentiated as long as the time interval overwhich it takes light to traverse the offset fiber 421 between the phasemodulator 419 and its reflection point 429 on the Sagnac loop 431 issmall compared to the characteristic time interval of the inputwaveform, which is inversely proportional to the characteristic waveformfrequency.

By using this characteritic the system may be used to transmitgeneralized analog waveforms by first integrating them prior to applyingthem to the phase modulator 419 and using the system which acts todifferentiate them back to their original form. The effectiveness ofthis approach depends upon appropriate offset of the phase modulator 419a process that can become increasingly difficult at very highfrequencies. The output waveform is shown at 431. The purpose of thephase modulator 415 and bias driver 417 is to offset any nonreciprocalphase drift between the counterpropagating light beams 425 and 427 dueto environmental effects or aging of system components. This may be doneby applying a sawtooth waveform via the bias driver 417 to the phasemodulator 415. The overall purpose of this method is to optimize theoutput signal of the receiver 401.

FIG. 5 illustrates two possible sawtooth approaches. In FIG. 5a thesawtooth has regions of positive (ramped) and negative (nearly vertical)slope corresponding to two net phase differences between thecounterpropagating light beams as they travelled in directions 425 and427 in FIG. 4. The major limitation of this approach is that if thestate of the data level does not change, the phase modulator or itsdriver will run out of dynamic range as is shown in FIG. 5b.

FIG. 6 illustrates how the waveform of FIG. 5 may be used to recreate adigital data stream. The waveforms of FIG. 6 include clock 601, data603, a positive slope sawtooth 605, a negative slope sawtooth 607 and acombination sawtooth 609. An input clock 601 is used to periodicallyreset the sawtoothed waveforms. In the first method the sawtooth 605 hasa positive slope. When the data is at the high level state, the inputdata waveform 603 is sawtooth modulated with the clock period. When thedata is in the lower state the modulation is turned off. The secondmethod uses a similar format, but with a negatively sloped sawtoothwaveform 607. The third approach is to use alternating positive andnegative sloped composite waveform 609 which depends upon the levelstate of the data 603 at each clock cycle.

FIG. 7 shows in block diagram form an electronics set that could be usedto format the input data. The input data 701 is fed into a datainterface box 703 that formats the output into a clock signal line 705and an output data stream into a data stream line 707. The clock signalline 705 is fed into an integrator reset circuit 709 while the datastream line 707 is fed into an integrator 711. The integrated outputfrom the integrator 711 is then fed into an amplifier 713 where it isamplified and buffered. The output of amplifier 713 is then fed into aphase modulator 715 which could correspond to the phase modulator 419 ofFIG. 4.

FIG. 8 illustrates a simple tap of a secure Sagnac interferometer basedcommunications system which consists of a beamsplitter 801 fused intothe fiber loop 803. The intruder has a fiber portion 805 to transportlight travelling in the counterclockwise direction, with respect to thesystem being infiltrated, along fiber loop 803 and a fiber portion 807to transport tapped light travelling in the clockwise direction, withrespect to the system being infiltrated, along fiber loop 803. Fiberportions 805 and 807 are connected to a beamsplitter 809. Beamsplitter809 is connected to a detector 811.

The system into which the intruder is tapping further comprises random(or pseudorandom) pathlength generator 813 along fiber loop 803. Randompathlength generator 813 has a symmetric point 815 on the other side offiber loop 803, with respect to the midpoint of the loop. The fiber loop803 extends into the boundary of a receiver 817.

The boundary of receiver 817 encompasses a light source 819 connectedthrough depolarizing element 820 to a central beamsplitter 821. Centralbeamsplitter 821 is in turn connected back into a detector 823. Atransmitter 825 is located along fiber loop 803 displaced from thecenter thereof with respect to central beamsplitter 821.

The dimension L_(A) indicates the shorter length from the location 801of the intruder tap to the central beamsplitter 821. The dimension L_(B)indicates the longer length from the location 801 of the intruder tap tothe central beamsplitter 821. The dimension L_(C) indicates the lengthfrom random pathlength generator 813, through the midpoint of loop 803to the symmetric point 815.

Once the intruder taped into fiber loop 803 and coupled thecounterpropagating light beams into his device, the counterpropagatinglight beams would then have to be pathlength matched to high accuracybefore the signal could be extracted.

For example, a conventional light emitting diode has a coherence lengthof about 35 microns. For a multikilometer system, the intruder would befaced with the prospect of matching multikilometer lengths to on theorder of 30-40 microns. In a system, shown by FIG. 8, the length offiber length 805 should be matched to the distance L_(A) and the fiberlength 807 should be matched to the distance L_(B) before the beams arecombined on the intruder's beamsplitter 809 if the beams are tointerfere to cause an amplitude modulated signal to result on thedetector 811. Matching kilometers lengths of fiber to distances on theorder of 100 microns is an extremely difficult and time consuming task.The situation is analogous to looking for a needle in a haystack, and inmany cases even the simplest system without the random pathlengthgenerator 813 may provide sufficient security.

For additional security the random pathlength generator 813 may be addedto the system . This has the effect of randomly (or more accuratelypseudorandomly) changing the pathlength of one of the fiber legs such asL_(B). L_(B) =L_(BO) +L.sub.(t) where L_(BO) is the nominal length ofthe fiber leg and L.sub.(t) is an arbitrary length function which isintroduced at characteristic time intervals. With random pathlengthgeneration, the system becomes much more secure. The situation isanalogous to randomly moving the needle throughout the haystack. Thepotential intruder is then faced with the formidable task of trying toachieve a very tight matching condition that is constantly changing. Thereceiver 817, however, resets quickly after a random pathlength chargeis introduced.

In the case of a random pathlength generator 813 located in the fiberloop, the Sagnac interferometer of the secure communication systemresets itself due to its self matching characteristics in a timeinterval given by L_(c) n/c where L_(c) is the length of the fiber loopbetween the random pathlength generator 813 and the symmetry point 815on the fiber loop 803 and opposite the central fiber beamsplitter 821.For a length L_(C) of 20 kilometers, the system resets itself in about10⁻⁴ seconds. Over this time interval information may not be transmittedto the receiver 817 as the resulting combined beams will not be mutuallycoherent due to the random pathlength introduction. If the randompathlength generator shifts the pathlength at a 100 Hz rate for the 20kilometer example, this would result in about a one per cent loss inpotential bandwidth. As an example, for the case where the randompathlength generator 813 is co-located with the transmitter station 825,signal drop out would occur over one of the timing intervals illustratedby FIG. 3. An important case is where the transmitter is located at thecenter of the loop 803. By locating the random pathlength generator 813at or near the center of the fiber loop 803 the reset time approacheszero as both counterpropagating light beams arrive simultaneously.

FIG. 9 shows a secure fiber optic communication system with a simplepassive intruder 901, the details of which are similar to thatillustrated by FIG. 8. As in FIG. 8, a receiver 903 encloses a lightsource 905, depolarizing element 907, central beamsplitter 909, anddetector 911. A fiber loop 913 connects receiver 903 with transmitter915, random pathlength generator 917 and passive intruder 901. Here therandom pathlength generator is placed in the center of the loop 913 as aspecial case of the more general description of FIG. 8. Note that thereset time and effect on the output receiver of the system becomevanishingly small as the random pathlength generator approaches the truecenter of the Sagnac loop.

An alternative approach to the use of random pathlength generator 917,yet insuring security against intrusion for a tap with pathlengthmatching by the intruder, is to use alarm systems based on optical powerdetectors. The purpose of these types of alarms is to (1) insure thatthe power level allowed the intruder is so low that useful data may notbe extracted and (2) to alert the user of the presence of an intrusionattempt. The figures that follow illustrate several of alarmconfigurations for the secure fiber optic communication system.

The system of FIG. 10 consists of a receiver unit 1001 and a transmitter1003 connected by the pair of fibers 1005. The receiver unit 1001consists of a light source 1007, a polarization scrambling ordepolarizing element 1009, a central fiber optic beamsplitter 1011 andan output detector 1013. The transmitter unit 1003 consists of apolarization scrambler 1015, a phase modulator 1017, the polarizationscrambler 1019 the alarm fiber beamsplitter 1021 which taps into one ofthe fibers 1005, the alarm output detectors 1023 and 1025 connecteddirectly to beamsplitter 1021, and the outputs 1027 and 1029 of alarmoutput detectors 1023 and 1025 respectively, the ratio transducer 1031connected to outputs 1027 and 1029, and alarm output 1033 from ratiotransducer 1031.

Light from the light source 1007 is coupled into the polarizationscrambler 1009 which reduces residual polarization dependence of thelight beam, The depolarized light beam is split by the central fiberoptic beamsplitter 1011 into the counterpropagating beams of lighttravelling in directions 1035 and 1037, The clockwise propagating beamof light propagates in direction 1035 and passes through the connectingfiber 1005 to the transmitter 1003. A portion of the light beampropagating in direction 1035 is tapped off onto the alarm detector1025, The tapped portion of the light beam falling onto the detector1025 generates an output signal that is used directly as the output 1029and as an input to the ratio transducer 1031, The remainder of theclockwise propagating light beam continues to circulate about the Sagnacloop of fibers 1005 passing through the polarization scrambler 1019, thephase modulator 1017 and the polarization scrambler 1015, The purpose ofthe polarization scramblers 1019 and 1015 before and after the phasemodulator 1017 is to reduce any polarization dependent effects that thephase modulator 1017 may have upon either of the counterpropagatinglight beams, The clockwise propagating light beam then returns via thefiber optic link 1005 to the central fiber optic beamsplitter 1011 whereit mixes with the counterclockwise propagating light beam of direction1037 to form the output amplitude modulated signal of the system that isdirected toward the output detector 1013. The counterclockwisepropagating light beam travelling in direction 1037 traverses the Sagnacloop formed by fiber optic link 1005 in the opposite direction passingthe same elements. The main difference is that when the alarm systembeamsplitter 1021 taps off a portion of the light of thecounterclockwise light beam, the tapped portion is directed to thedetector 1023. The resultant output from the detector 1023 is monitoreddirectly and also used as an input to the ratio transducer 1031. Thereare a number of advantages to this alarm system approach and the Sagnacsecure communication system when configured in this manner. The firstadvantage is that barring environmenal fluctuation in the transmissiveproperties of the optical components in the Sagnac loop, the ratio ofoptical power between the light beams circulating about the loop remainsfixed. This results in a very stable signal and the tolerances on theresulting ratioed alarm output 1033 can be maintained to high levels.This makes it difficult to intrude on the exposed portions of the Sagnacloop without triggering a properly configured ratioed alarm system. Thesecond major feature of this configuration is that since the fiberbeamsplitter 1021 is a single element and highly reciprocal, anyenvironmental fluctuations in this element would not be expected tostrongly effect the resulting output ratioed signal 1033.

The limitations of the configuration shown in FIG. 10 are mainly due toenvironmentally induced loss changes in components in the Sagnac loopformed by fiber optic link 1005. In particular the phase modulator 1017,which may be a pigtailed integrated optical device, may have opticallosses that are temperature, pressure, polarization, and wavelengthdependent. To circumvent this limitation the configuration shown in FIG.11 may be used.

In FIG. 11, the receiver 1101 is identical to the receiver 1001 of FIG.10. The transmitter 1103 is connected to the receiver 1101 by the fibers1105. The transmitter section 1103 contains a dual alarm system tap. Afiber beamsplitter 1107, a wavelength passband filter 1109, apolarization scrambling element 1111, the phase modulator 1113, thepolarization scrambling element 1115, and the fiber beamsplitter 1117are located along the main Sagnac light path. The alarm system detectors1119 and 1121 are connected to beamsplitters 1107 and 1117,respectively. Detector 1119 has an output 1123. Detector 1121 has anoutput 1125. Outputs 1123 and 1125 are connected to a ratio transducer1127 which has a ratio output 1129.

The operation of the secure fiber optic communication link as shown inFIG. 11 is as follows. The receiver 1101 generates thecounterpropagating light beams which travel in direction 1131 and 1133.The clockwise propagating light beam circulates about the Sagnac loop indirection 1131 and after entering the transmitter 1103 passes the firstfiber beamsplitter 1107, the first alarm system tap. A portion of theclockwise propagating light beam is split off or coupled by the tap 1107and propagates to the alarm system detector 1119. The output from thedetector 1119 both constitutes the direct alarm system output 1123 andas an input fed into the ratio transducer 1127. The main portion of theclockwise propagating light beam continues to circulate about the Sagnacloop and next passes the wavelength passband filter 1109. The functionof the wavelength passband filter is to ensure that the only wavelengthsof light that will be permitted to propagate through the Sagnac loop arethe ones that can be monitored by the alarm system detectors 1119 and1121. The clockwise propagating light beam then passes the polarizationscrambling element 1111, phase modulator 1113 and polarizationscrambling element 1115 before reaching the second fiber opticbeamsplitter 1117. In this configuration a portion of the clockwisepropagating light beam is tapped off to the terminated fiber optic end1135 which is arranged to avoid back reflection. The main portion of theclockwise propagating light beam then returns to the receiver 1101. Thecounterclockwise circulating optical light beam travelling in direction1133 traces a path about the Sagnac loop in the opposite direction. Thefiber beamsplitter 1117 taps a portion of the counterclockwisepropagating light beam 1133 off to the detector 1121. The output fromthe detector 1121 is both used as a direct output 1125 and as an inputfed into the ratio alarm circuit 1127. When the counterclockwisepropagating light beam reaches the fiber optic beamsplitter alarm systemtap 1107 a portion of the light beam is split off and directed to aterminated end 1137 configured to avoid back reflection. The mainportion of the counterclockwise propagating light beam then returns tothe receiver 1101. In place of the terminated ends 1135 and 1137, aredundant alarm system having additional direct detection and ratioedalarm outputs could be used. This redundent systems stability wouldhowever be subject to environmentally induced loss fluctuations of theelements 1115, 1113, 1111, and 1109, and which could limit its utility.

The alarm systems described in association with FIGS. 10 and 11 aretotal power dependent alarm systems. This means that they are configuredto monitor fluctuations in optical power circulating about the Sagnacloop. An alterative approach includes the monitoring of the spectralcontent of the light circulating about the Sagnac loop. FIG. 12illustrates one configuration of such a spectral alarm system. Here thereceiver 1201 and connecting fibers 1205 are similar to those describedin the earlier FIGS. The transmitter 1203 is similar to that oftransmitter 1003 of FIG. 10 except that here there are two pairs ofratioed alarm systems 1207 and 1209.

The ratioed alarm system 1207 is designed to use a wavelength divisionmultiplexing fiber beamsplitter 1211 that covers a portion of thespectrum of the light circulating through the fiber loop. The ratioedalarm system 1209 is designed to use a wavelength division multiplexingfiber beamsplitter 1213 that is designed to cover another portion of thespectrum of the light circulating through the fiber loop. Together thetwo ratioed alarm systems 1207 and 1209 are designed to cover the entirespectrum of light circulating through the system. They are also designedto be overlapped on only a portion of the total spectrum. This presentsthe intruder with the prospect of having to spectrally control a tap aswell as total power.

FIG. 13 illustrates a system based on ratioed spectral alarms. Thereceiver 1301 is connected to the transmitter section via the fiberoptic cable 1305. The receiver unit 1301 is similar to those describedin association with FIGS. 10 to 12, as are the connecting fibers 1305.The transmitter section 1303 combines the bypass features described inassociation with FIG. 11 with the ratioed spectral alarm systems of FIG.12. In particular the receiver 1301 generates the counterpropagatinglight beams which travel in direction 1307 and 1309. The clockwisepropagating light beam of direction 1307 circulates to the transmitter1303, and upon entering passes the fiber beamsplitter 1311 of the alarmsystem tap where a portion of the light beam is coupled off and directedpast the ratio spectral alarm units 1313 and 1315 whose boundaries areset off by dashed lines. Spectral alarm units 1313 and 1315 are set upto selectively split off in wavelength bands centered about wavelength 1and wavelength 2 and are designed to cover the entire spectrum of lightpropagating through the system. The residual light beam that passes bythe alarm systems 1313 and 1315 then passes through the fiberbeamsplitter 1317 of the alarm system tap where the light beam is againsplit into a portion being directed to a terminated end 1319 and aportion directed via the fibers 1305 back to the receiver 1301. Ingeneral, the light that is tapped off by beamsplitter 1311 and 1317 andpropagates through the alarm system bypass to return to the receiver canbe made very small. The beamsplitter 1311 and 1317 will generally bedesigned to tap a small fraction of the light circulating about theSagnac loop. In addition, the spectral ratioed alarm systems 1313 and1315 can be arranged to tap off most of the optical power directedthrough the bypass. The main portion of the clockwise propagating lightbeam that does not pass through the alarm bypass output part ofbeamsplitter 1311 is directed through the wavelength passband filter1321 and the phase modulator 1323. When passing the beamsplitter 1317, aportion of the clockwise propagating light beam returns to the receiver1301 and a portion is split off to the terminated end 1319. Thecounterclockwise light beam of direction 1309 propagates about the loopin the opposite direction in a similar manner through the same elements.One port of beamsplitter 1311 ends in a termination 1325 so that theportion of the counterclockwise propagating light beam that circulatesthrough the system will exit the system instead of returning to thereceiver 1301.

FIG. 14 illustrates a system based on the used of multiple wavelengthlight sources, multiple ratioed alarm systems and multiple wavelengthSagnac loops to increase the difficulty of intrusion attempts. Thesystem consist of the receiver 1401, connecting fibers 1405 and thetransmitter unit 1403. The receiver unit 1401 consists of three lightsources 1407, 1409 and 1411 operating at three distinct wavelengthslamda 1, lamda 2 and lamda 3 respectively, the wavelength divisionmultiplexing beamsplitters 1413 and 1415, the depolarizing element 1417,the central fiber optic beamsplitter 1419 and the output detector 1421.The transmitter 1403 consists of the wavelength division mulitplexingfiber beamsplitters 1423, 1425, 1427, 1429, 1431, 1433, 1435, and 1437,the detectors for the ratioed alarm system 1439, 1441, 1443, 1445, 1447,1449, 1451 and 1453, the phase modulator 1455 and random pathlengthgenerator 1457.

FIG. 14 is a diagram of a multispectral alarm system for the Sagnacinterferometer based secure fiber optic communication system. Light fromthree light sources 1407, 1409 and 1411 operating at the wavelengthslamda 1, lamda 2 and lamda 3 are coupled into the fiber depolarizer1417. Light from the light source 1411 is coupled with light from thelight source 1409 via wavelength division beamsplitter 1413. Light fromthe light source 1407 is coupled with light from wavelength divisionbeamsplitter 1413 at wavelength division beamsplitter 1415. Theresulting three wavelength light beam is then coupled to the fiberdepolarizer 1417.

The resulting three color light beam is split into thecounterpropagating light beams having a clockwise direction 1459 and acounterclockwise direction 1461 by the central beamsplitter 1419. As theclockwise propagating light beam of direction 1459 passes the wavelengthdivision multiplexing beamsplitter 1423, a portion of the light beamhaving the third wavelength lamda 3 is split off onto detector 1441.Then as the clockwise propagating light beam passes the wavelengthdivision mutiplexing beamsplitter 1425, a portion of the light havingthe first wavelength lamda 1 is split off and propagates onto thedetector 1445.

The remainder of the clockwise propagating light beam of direction 1459proceeds to the wavelength division multiplexing beamsplitter 1427. Thewavelength division multiplexing beamsplitter 1427 selectively routesthe clockwise propagating light beam light having a wavelength lamda 3down path 1463, and is routed back into the Sagnac loop by thewavelength division multiplexing element 1433. The light beam having awavelength lamda 3 then passes the wavelength division multiplexingbeamsplitter 1435 and reaches the wavelength division multiplexingbeamsplitter 1437 where a portion of the light beam is directed to thealarm detector 1453. The remainder of this light beam having wavelengthlamda 3 returns to the receiver 1401 via the fiber 1405, and thenreaches the central fiber optic beamsplitter 1419 where it recombineswith the counterclockwise counterpropagating portion of the light beamhaving a wavelength lamda 3 of direction 1461. Since data is notimpresses on the lamda 3 light beams the resultant signal from thecombined beams at this wavelength corresponds to a slowly varyingambient light level at this wavelength.

Referring back to the upper right portion of transmitter 1403, lighthaving wavelengths lamda 1 and lamda 2 leave wavelength divisionmultiplexing beamsplitter 1427 and propagate to wavelength divisionmultiplexing beamsplitter 1429 where light having wavelength lamda 2 isrouted away from light having a wavelength lamda 1. The light beamhaving wavelength lamda 2 is routed in direction 1467 through the phasemodulator 1455 and through a random pathlength generator 1457 beforebeing returned to the Sagnac loop by the wavelength divisionmultiplexing beamsplitter 1431. The light beam having a wavelength lamda2 then passes the wavelength division multiplexing beamsplitters 1435and 1437 as it returns to the receiver 1401. Inside the receiver 1401the lamda 2 light beam falls onto the central fiber optic beamsplitter1419 where it combines with the circulating portion of thecounterclockwise counterpropagating lamda 2 light beam. Since the phasemodulator 1455 has been used to impress data onto thesecounterpropagating lamda 2 light beams in terms of a relative phasedifference, the two light beams interfere and recreate the impresseddata in terms of an amplitude modulated signal that is directed towardthe output detector 1421.

Again, back to the upper right hand portion of the transmitter 1403,light having a wavelength lamda 1 propagates in direction 1469 as itcirculates about the bypass to the wavelength division multiplexingelement 1431 where it is recoupled to the Sagnac loop. The lamda 1 lightbeam then passes the wavelength division multiplexing element 1433, andreaches the wavelength division multiplexing beamsplitter 1435. At thispoint, a portion of the lamda 1 light beam is tapped off to the alarmdetector 1449. The remainder of the lamda 1 light beam passes thewavelength division multiplexing beamsplitter 1437 and returns to thecentral fiber optic beamsplitter 1419. There the clockwise propagatinglamda 1 light beam recombines with the portion of the counterclockwisecounterpropagating lamda 1 light beam. Like the lamda 3 light beams,these beams again do not contain information and are strictly in placefor alarm detection purposes. The counterclockwise light beam ofdirection 1461 propagates through the system in the opposite directionin analogous fashion. By selecting the "guard band" wavelengths higherand lower than the carrier wavelength, intrusion attempts are made moredifficult. The Sagnac interferometer approach offers the adavantage ofbeing able to employ ratio alarms that can be used in redundant fashionand with great stability to enhance the overall security of the system.

FIG. 15 shows a multispectral alarm system that is essentially identicalto that of FIG. 14. The major difference is that the ratio alarm systemshave been moved from the transmitter section to the receiver section. Inparticular the receiver 1501 and transmitter 1503 are identical to thoseof FIG. 14. Now, the receiver 1501 contains the ratio alarms 1507 and1509. The transmitter 1503 here has its ratio alarm systems removed. Thefibers 1505 interconnect the transmitter 1503 and receiver 1501. Thechange in location of the ratio alarm systems does not affect operation,and operation is identical to that described for FIG. 15.

An alternative multispectral alarm approach is shown in FIG. 16. Ratherthan use multiple distinct light sources, a single broadband lightsource is employed. The broadband light coupled into the systempropagates around a single Sagnac loop. The alarm system consists of aseries of appropriately spaced wavelength division multiplexingbeamsplitters which collectively are capable of diverting wavelengths oflight covering the entire spectral range of the broadband light source.This design is used in a ratio alarm format to take advantage of theself referencing properties of the Sagnac interferometer.

The system consists of a receiver 1601 and a transmitter 1603interconnected by a pair of optical fibers 1605. The receiver 1601consists of a broadband light source 1607 a depolarizing element 1609, acentral fiber optic beamsplitter 1611, a series of several ratioedspectral alarm systems represented by a bilateral set or pair ofspectral alarm systems 1613 through 1615, and an output detector 1617.The transmitter 1603 consists of the phase modulator 1619 that is offsetfrom the center of the Sagnac loop with respect to central beamsplitter1611 and a passband filter 1621 that is optimized for transmission ofwavelenghts output by the broadband light source 1607 and handled by themultispectral ratio alarm system, and optimized for attentuation of allother wavelengths. The passband filter 1621 may be, for example, andispersive optical filter. An dispersive optical filter is made of afiber with highly dispersive core and which has cladding regions whichchange waveguiding characterisitics as a function of wavelength.Alternatively, any of several optical passband filters described in theliterature may be used for passband filter 1621. The operation of thissystem is similar to that described in association with the earlierfigures. The ratioed alarm system as configured in the embodiment ofFIG. 16 has the advantage of monitoring the spectral content of verynarrow regions of the light circulating in the Sagnac loop. Potentiallyan increasingly larger number of these systems could be used resultingin concomitantly increasing security. An additional feature of thissystem includes the ability to monitor the collective spectral contentof the light source being monitored and ratioed by the alarm system.Since the Sagnac loop provides a reference, the ratio alarms will not betriggered by spectral drift of the light source, The system is alsoindependent of the length of the link since a ratio established afterconnection remains fixed,

FIG. 17 illustrates a system similar to that shown in FIG. 16, but herethe multispectral ratioed alarm system moved to the transmitter section.The receiver 1701, transmitter 1703, optical fibers 1705, andmultispectral light source 1707 are similar to receiver 1201,transmitter 1203, optical fiber 1205, and multispectral light source1607 described in FIG. 16. Transmitter 1703 does, however, include theaddition of multispectral ratioed alarms 1709 and 1711.

FIG. 18 illustrates the schematic of a system similar to that of FIG 17.In FIG. 18, the multispectral alarm has been replaced by directionaldispersive elements and detector arrays. The receiver 1801, transmitter1803 optical fibers 1805 and multispectral light source 1809 are similarto receiver 1701, transmitter 1703, optical fibers 1705 andmultispectral light source 1707 of FIG. 17, Here, transmitter 1803includes the directional dispersive elements 1811 and 1813, detectorarray elements 1821 and 1823 and their outputs 1831 and 1833 that areratioed, as well as a phase modulator 1851 and wavelength passbandfilter 1853.

The action of a single dispersive element such as dispersive element1811 in combination with a detector array such as detector array 1821 isto monitor the spectral content of the light source 1809. Unexpectedchanges in the light content due to an intrusion would activate thealarm. Directional dispersive elements 1811 and 1813, such as gratingelements, cause the light spectrum from each of the counterpropagatinglight beams to become spread onto each of the detector arrays 1821 and1823, respectively and the result available for ratioed referencing. Inthis manner very slight perturbations to any portion of the spectralcontent of light circulating through the system could be detected.

FIG. 19 illustrates one embodiment of a full duplex multispectral securefiber optic communication system. This embodiment uses one of thesimplest and most effective methods for a multispectral alarm system. Atransceiver unit 1901 is fiber optically connected to a transceiver unit1903. Light from a light source 1905 with a spectral output centeredabout lamda 1 is injected into an optical fiber 1907. Light passesthrough a polarization scrambling element 1909 on its way tobeamsplitter 1911. Beamsplitter 1911 splits the polarization scrambledlight into counterpropagating light beams which travel in directions1913 and 1915. Optical beamsplitter 1911 is designed to split theoptical power of the light beams centered about lamda 1 approximatelyequally. The clockwise light beam passes a wavelength divisionmultiplexing element 1917 that is fabricated to pass light centeredabout wavelength lamda 1 without cross coupling. This process could beaccomplished by using a specially fabricated fiber beamsplitter or anyother wavelength division multiplexing element such as those based ondispersive elements. After passing through the wavelength divisionmultiplexing element 1917 the clockwise propagating light beam enters oninterlink fiber 1919 that acts to connect the transceiver 1901 to thetransceiver 1903. After entering transceiver 1903 the light beam iscross coupled by the wavelength division multiplexing element 1921 whichis designed to cross couple light and route centered about wavelengthlamda 1. The routed light beam then passes a tap and detector unit 1923which taps off a portion of the light beam for alarm system purposes.The routed light beam then passes through a random pathlength generator1925 which ideally may be positioned near the center of the Sagnac loop,and is used if necessary to provide additional security as earlierdescribed. The clockwise propagating light beam then passes through aphase modulator 1927 which is used to impress phase information onto theclockwise propagating lamda 1 wavelength light beam for datatransmission. The clockwise propagating lamda 1 wavelength light beamthen passes through a tap and detector 1929 which is used to monitor thelight intensity of the counterpropagating lamda 1 wavelength light beam.The clockwise propagating lamda 1 wavelength light is then cross coupledby the wavelength division multiplexed element 1931 and routed into theinterconnecting fiber 1933. The light beam is directed byinterconnecting fiber 1933 back to transceiver 1901. The clockwisepropagating lamda 1 wavelength light beam then passes through awavelength division multiplexed element 1935 and back to the fiberbeamsplitter 1911.

The counterclockwise lamda 1 wavelength light beam travelling indrection 1915 traverses the optical path just described for theclockwise propagating lamda 1 wavelength light beam, but in thecounterclockwise direction. The major distinction in thecounterclockwise case is that the tap/detector element 1929 is used tomonitor the light beam. The clockwise and counterclockwise lamda 1 lightbeams interferometrically combine on the beamsplitter 1911 and theresultant modulated signal falls onto an output detector 1937.

The light source 1955 generates a light beam centered about wavelengthlamda 2 that is coupled into an optical fiber 1957. After passing apolarization scrambling element 1959 the lamda 2 light beam propagatesto fiber beamsplitter 1961. The light beam is split by fiberbeamsplitter 1961 into counterpropagaging light beams. Fiberbeamsplitter 1961 is designed to split light centered about a wavelengthlamda 2 approximately equally for optimum performance. Thecounterclockwise propagating lamda 2 light beam passes throughwavelength division multiplexing element 1921 and the fiber 1919 beforebeing cross coupled and routed away by wavelength division multiplexedunit 1917. The routed counterclockwise propagating lamda 2 light beam isthen monitored by a tap/detector 1963 for light intensity level and thenpasses through a random pathlength generator 1965, a phase modulator1967, and by a tap/detector 1969. The wavelength division multiplexsplitter 1935 cross couples the counterclockwise propagating lamda 2light beam into the fiber 1933 which directs it back to transceiver 1903where it passes through the wavelength division multiplexed splitter1931 and onto the fiber beamsplitter 1961. The clockwise propagatinglamda 2 wavelength light beam traverses the optical path ofcounterclockwise propagating lamda 2 wavelength light beam in theopposite direction with the tap/detector element 1969 monitoring itsintensity level. The two beams recombine on fiber beamsplitter 1961 andthe resultant signal falls onto an output detector 1971.

FIG. 20 is similar to FIG. 19 except that tap/detectors 1963, 1969,1923, and 1929 have been replaced by the ratioed alarms 2001 and 2003that use the single fiber beamsplitter taps 2005 and 2007 instead ofdual taps.

FIG. 21 shows a block diagram of the electronics supporting the fullduplex multispectral secure fiber optic communication system for thetransmitter section, a pair of communication lines 2103 and 2105 areavailable to the optical circuit of FIG. 21. A wavelength divisionmulitplexing element 2107 is located along communication line 2103, anda wavelength division multiplexing element 2109 is located alongcommunication line 2105, the outputs from a tap 2111 and detector 2113and from a tap 2115 and detector 2117 are fed into the ratio/dividercircuitry 2119 whose output is fed to the normalization circuitry block2121. The outputs from detectors 2113 and 2117 are fed directly into thenormalization circuit blocks 2123 and 2125, respectively. When thesystem is activiated, these normalization circuit blocks 2121, 2123 and2125 go through an initialization process wherein the power levels onthe detectors 2113 and 2117 are determined. This determined value isused to normalize future outputs as well as their ratio. The outputsfrom the normalization circuit blocks 2121, 2123 and 2125 are fed intoan alarm controller 2127 which shuts down the system by turning off thedata stream through connection with a data formatter 2129 if thedeviation of the power level from the detectors 2113 and 2117 or theirratio exceeds preset trigger values.

A random pathlength generator 2131 and a phase modulator 2133 areconnected in series between taps 2111 and 2115. Random pathlengthgenerator 2131 is electrically connected to a random pathlengthgenerator control circuiting block 2135. Phase modulator 2133 iselectrically connected to data formatter 2129. The data formatter 2129is electrically connected to block 2135. The data is input to the systemvia an input port 2137 of the data formatter 2129. When the alarmcontroller 2127 detects that trigger values have been exceeded it shutsdown the data formatter 2129 which shuts down the data stream 2137.

FIG. 22 illustrates a block diagram of a possible set of supportelectronics for the receiver section of the system. The main features ofFIG. 22 are the normalization and power controller for the light source.The purpose of this system is to hold the output power coupled into thesystem as stable as possible enabling the alarm system tolerance to betight. Referring to FIG. 22, a pair of incoming fiber optic lines 2201and 2203 enter the FIG. in dashed format from the left-most side. Fiberoptic line 2201 and 2203 extend to wavelength division multiplexingelements 2207 and 2209, respectively. Wavelength division multiplexingelement 2207 is connected to a detector 2211 on the left, and to a tap2113 and detector 2215 on the right. Similarly, a wavelength divisionmultiplexing element 2209 is connected to a tap 2217 and detector 2219on its right extension. In between taps 2217 and 2213 is a randompathlength generator 2221 and a phase modulator 2223 connected inseries.

Wavelength division multiplexing elements 2207 and 2209 are also eachconnected to one of four ports of a central beamsplitter 2231. The thirdand fourth parts of central beamsplitter 2231 are connected to a lightsource 2233 and a detector 2235, respectively. Detector 2235 connectsinto a data formatter 3341. Data formatter 2241 connects to a signalpower monitor 2245. Both the signal power monitor 2245 and light source2233 are connected into a power controller 2251. Power controller 2251is also connected back into detector 2111.

Power stability is accomplished by having detector 2211 monitor thesmall amount of light at wavelength to be modulated cross coupled by thewavelength division multiplexing unit 2207. The output of detector 2211is then fed into a power controller 2251 which adjusts the current tothe light source 2233 to control and stabilize the light power injectedinto the system. Another alarm system contained within the receivingsection shown in FIG. 22 consists of signal power monitor 2245 whichmonitors the signal level of the data received on the detector 2235.This is accomplished directly or in combination with the output dataformatter 2241. In the event that the output signal level deviates fromexpected values, the signal controller would deactivate the light source2233 via the power controller 2251.

Another embodiment of the full duplex mulitspectral secure fiber opticcommunication system is shown in FIG. 23. A pair of transceivers 2301and 2303 are shown connected by a pair of optical fiber 2305 and 2307.Beginning at the left, transceiver 2301 contains light source 2311connected to one of four ports of a central beamsplitter 2313. Adetector 2315 is connected to beamsplitter 2313. The other two parts ofbeamsplitter 2315 are connected to the optic fibers 2305 and 2307. Thethird and fourth ports of beamsplitters 2313 are connected to taps 2321and 2323, respectively. Tap 2321 is connected to detectors 2325 and 2327while tap 2323 is connected to detectors 2329 and 2331. A seriesconnection is established within transceiver 2301 between optical fiber2305 and 2307 beginning at fiber 2305 and comprising a wavelengthdivision mulitplexer 2341, a random pathlength generator 2343, phasemodulator 2345 and wavelength division multiplexer 2347.

Similarly, transceiver 2303 contains light source 2361 connected to oneof four ports of a central beamsplitter 2363. A detector 2365 isconnected to the beamsplitter 2363. The other two ports of abeamsplitter 2363 are connected to the optic fibers 2305 and 2307. Thethird and fourth ports of beamsplitter 2363 are connected to taps 2371and 2373, respectively. Tap 2371 is connected to detector 2375 and 2377while tap 2373 is connected to detectors 2379 and 2381. A seriesconnection is also established within transceiver 2303 between opticalfiber 2305 and 2307 beginning at fiber 2305 and comprising a wavelengthdivision multiplexer 2391, a random pathlength generator 2393, phasemodulator 2395 and wavelength division mulitplexer 2397.

Here, in FIG. 23, instead of the detector tap combinations of FIG. 20,alarm systems are built around the fiber taps 2321, 2371, 2323 and 2373of transceivers 2301 and 2303. The taps 2321 and 2323 are used tomonitor the output from the light source 2311, the performance of thebeamsplitter 2313 and the light power of the counterpropagating lightbeams generated by the light source 2311 through the Sagnac loop formedbetween transceiver 2301 and 2303. The ratio of the light power ondetector 2327 and 2331 can be used to normalize any deviations from astable split of the optical power by the beamsplitter 2313. The sum ofthe power on detector 2327 and 2331 monitors the total light injectedinto the system by the light source 2311. The detectors 2325 and 2329can be used for direct detection of power level in the Sagnac loop.Their ratio can be used to detect attempts to make unauthorized singlesided taps. Similarly, the detectors 2375, 2377, 2379 and 2381 are usedin combination with the taps 2371 and 2373 to monitor the Sagnac loopdriven by the light source 2361.

FIG. 24 is a block diagram of the alarm electronics that would beassociated with the alarm system of FIG. 23. A light source 2401 isconnected to one port of a central beamsplitter 2403, which is in turnconnected to a detector 2405 similar to those illustrated in earlierFIG's. A pair of taps 2407 and 2409 are each connected to the opticalfibers 2411 and 2413, respectively. Tap 2407 is connected to detector2415 which has an output A and to detector 2417 which has an output B.Similarly, tap 2409 is connected to a detector 2419 which has an outputC and detector 2421 which has an output D. The outputs are so labelledto avoid drawing the connection lines on FIG. 24, whose cross connectionwould complicate the drawing.

Light source 2401 is connected to a light source driver power control2425. Light source driver power control 2425 is in turn connected to analarm system controller 2435. Alarm system controller 2435 receivesinputs from and is connected to ratio direct detection circuits 2451 and2453 and ratio sum detection circuits 2455 and 2457. Ratio directdetection circuits 2451 receives inputs A and B from detectors 2415 and2417, respectively while ratio direct detection circuit 2453 receivesinputs C and D from detectors 2419 and 2421, respectively. Ratio sumdetection circuit 2455 receives inputs B and D from detectors 2417 and2421, respectively while ratio sum detection circuit 2457 receivesinputs A and C from detectors 2415 and 2419, respectively.

The alarm system controller 2435 stabilizes the light source 2401 outputvia the light source driver/power controller 2425. If the power levelson the detectors 2415, 2417, 2419 or 2421 fall outside of the expectedlevels, the alarm system controller 2435 shuts down the light source2401 and may be used to activate acoustic or other electronic alarms.Many multiple redundant variations of alarm sceanarios could beprogrammed into the controller. This would improve fault tolerance. Thesystems of FIGS. 20-24 could be used in conjunction with all securecommunication applications compatible with the usage of single modeoptical fiber including terrestial telecommunications and computernetworks.

FIG. 25 illustrates an interferometric alarm system. It consists of areceiver section 2501, and a transmitter section 2503 a connecting fiberpair 2505. The receiver section 2501 consists of the light source 2507,input output beamsplitter 2509, fiber depolarizer 2511, fiber polarizer2513, fiber depolarizer 2515, central fiber optic beamsplitter 2517,phase modulator 2519, system output detector 2521, light sourcemonitoring detector 2523, interferometric alarm system detector 2525,harmonic analizer 2527, alarm threshold detector 2531, oscillator 2529,and light source driver 2533. Transmitter section 2503 includes passbandfilter 2539 and phase modulator 2541.

The operation of the device illustrated by FIG. 25 is as follows. Lightfrom the light source 2507 is coupled into one leg of the input/outputfiber beamsplitter 2509. One portion of this light beam is fed into thelight source monitoring detector 2523. The output from the light sourcemonitoring detector 2523 is fed to the light source controller 2533 andused to stabilize the output of the light source 2507. The output of thelight source detector 2523 is also fed into the harmonic analizer 2527and is used to provide a signal level reference. The signal is comparedto an initialized alarm system output signal established during turn onby the harmonic signal analyzer. The other portion of the light beam isdepolarized by the depolarizing element 2511 which may be a Lyot fiberdepolarizer. After depolarization, the light beam enters the polarizingelement 2513 which establishes a single polarization state for a highlyreciprocal system in analogy to the techniques employed for fiber opticgyroscopes. After passing through the polarizing element 2513 the lightbeam is once again depolarized by the depolarizing element 2515. Thelight beam then enters the central fiber optic beamsplitter 2517 whereit is split into counterpropagating light beams. The clockwisepropagating light beam proceeds in direction 2543 while thecounterclockwise light beam proceeds in direction 2545. The clockwisepropagating light beam circulates through the Sagnac loop through thewavelength passband filter 2539. The wavelength passband filter 2539 isdesigned to attenuate light outside the detection band of the alarmsystem. The clockwise propagating light beam then proceeds through thephase modulator 2541. Phase modulatoar 2541 is used to impress data ontoboth of the counterpropagating light beams in the form of both ofrelative phase differences. Phase modulator 2519 is used to generate areference signal for the interferometric alarm system by modulating therelative phase between the counterpropagating light beams. The referencesignal frequency impressed by phase modulator 2519 is chosen so as notto interfere with the transmitted data. As an example, this choice ofnon interfering frequencies may be performed by using a reference signalfrequency that is much lower than the tranmitted signal frequency. Thepurpose of the alarm system formed by the phase modulator 2519, theharmonic analizer 2527, the oscillator 2529 is to prevent successfulsophisticated intrusions onto the system. In particular intrusion systemthat tap light from the Sagnac loop and attempt to compensate byreinjecting light into the system would be greatly complicated by thisalarm. The counterclockwise counterpropagating beam of light ofdirection 2545 circulates about the Sagnac loop in the oppositedirection. After cycling through the Sagnac loop in opposite directionsthe two light beams recombine on the central fiber optic beamsplitter2517. The recombined light beams interfere resulting in amplitudemodulated signals that include the transmitted data and the alarm systemreference. The portion of the signal that is directed to the outputdetector 2521 is used to reconstruct the data stream. The other portionof the signal is directed back through the depolarizer 2515, polarizer2513 and depolarizer 2511 to the beamsplitter 2509 and the output alarmdetector 2525. Note that the depolarizer 2511 could be moved from theposition shown in FIG. 25 to a position located between the light source2507 and the input output beamsplitter 2509. This procedure would resultin the advantage of an increase in alarm system signal level which inturn would depend on the intrinsic loss within the fiber depolarizer2511. The output of the alarm detector 2525 is then fed into theharmonic analyser 2527. The harmonic analyser 2527 is also connected tothe tunable oscillator 2529 that is used to drive the phase modulator2519. The function of the harmonic analyser 2527 is to measure the powerof the counterpropagating light beams in the Sagnac loop as theycirculate. Since only the light beams that interfere coherently andcarry the phase modulated signal produced by the modulator 2519 generatethe alarm signal, many types of intrusion are thus complicated. Theoutput from the harmonic analyser 2527 is feed into an alarm system 2531which monitors changes in the alarm signal level. If the threshold ofthe alarm system 2531 is exceeded, a signal is sent to the light sourcecontroller 2533 that shuts down the light source 2507 output. This alarmsystem within the receiver of FIG 25 could be used in combination withany of the several other alarm system described for the other FIGS.above to form a very secure communication system.

All of the above system described secure fiber optic communicationsystems whose length is limited by fiber attenuation. To achieve longerlengths one could put these systems back to back. The disadvantage ofthis approach is that the data comes out electrically and must beguarded. Ideally, one would like an all optical repeater which does notcompromise security or need to be guarded. Several methods of doing thisare described in the following FIGS.

FIG. 26 shows an all optical repeater embedded into a basic Sagnacinterferometer based secure communication system.

A light source 2601 is connected to the first part of a beamsplitter2603. Beasmplitter 2603 is connected back into a detector 2605. Thesecond port of the third and fourth ports of beamsplitter 2603 form aSagnac loop 2607 series connection between a random pathlength generator2609, phase modulator 2611 and a secure repeater station 2613. Withinsecure repeater station 2613, diode arrays 2621 and 2623 are present toprovide light pumping of a fiber amplifier. The section of fiber 2625between diode arrays 2621 and 2623 is a doped fiber used to amplify thelight beams circulating about a Sagnac loop formed between the seriesconnections between parts 3 and 4 of beamsplitter 2603. Laser diodearrays 2621 and 2623 are used to side pump the doped fiber regions sothat light circulating through them is amplified. As a specific examplethe laser diode array could consist of GaAs based light sources emittingin the 0.8 to 0.9 micron wavelength region. The region of fiber beingpumped could be doped with trace materials such as erbium which absorbstrongly in this region. The resultant excited states for this examplewould then act as an optical amplifier for light in the longer 1.3 to1.5 micron region.

FIG. 27 is similar to FIG. 26. A light source 2701 is connected to thefirst port of beamsplitter 2703. The second port of beamsplitter 2703 isconnected back into a detector 2705. A Sagnac fiber loop 2707 makes aseries connection between ports 3 and 4 of beamsplitter 2703, but alsois made to pass through secure repeater station 2713 twice, both beforeand after making series connection with random pathlength generator 2709and phase modulator 2711. Diode arrays 2721 and 2723 are within repeatorstation 2713.

In FIG. 27 the fiber 2707 is used for the optical link to support thefiber optic loop 2707. FIG. 28 is similar to FIG. 27, except that dualcore optical fiber is used. A light source 2801 is connected to thefirst port of a beamsplitter 2803. A second port of beamsplitter 2803 isconnected to a detector 2805. The third and fourth ports of beamsplitter2803 is connected to two ports of a dual core optical fiber 2807. Dualcore optical fiber 2807 extends through a boundary demarking a receiversection 2809 before extending through repeater station 2813 to reachtransmitter section 2821. Within transmitter secion 2821, dual coreoptical fiber 2807 form a series connection between their dual coresthrough a random pathlength generator 2831, and phase modulator 2833.

Within the repeator station 2813 is a pair of diode arrays 2851 and 2853surrounding dual core fiber 2807. The region 2855 of the dual core fiber2807 is doped so that it acts as a fiber amplifier. The closeproximities of the paths within dual core fiber 2807 causesenvironmental effects to become minimized. The use of dual core fibersubstantially increase the complexities, and therefore enhances thedifficulty associated with unauthorized entry into the secure system. Inaddition, the number of individual fibers required to support the systemwould be reduced.

The details of the side pumped doped fiber amplifier section, are shownin FIG. 29. Fiber optic taps 2901 and 2903 are placed on both sides ofthe amplifying section of fiber 2905 so that the light levels of theincoming and outgoing light beams may be measured by the detectors 2911,2913, 2915 and 2917 having outputs A, B, C, and D, respectively.Detector outputs A, B, C, and D are fed into a laser diode arraycontroller 2921 that adjusts the laser diode array pump 2925 level tomaintain a constant level of amplification. They are also used tosupport an alarm system placed about the amplifying fiber.

FIG. 30 shows an alternative configuration for the repeater station thatis easier to implement. Again, taps 3001 and 3003 on line 3005 connectto detectors 3011, 3013, 3015 and 3017 having outputs A, B, C, and D,respectively. A single laser diode 3051 is pigtailed in to optical fiber3005 and spliced into a wavelength division multiplexing fiberbeamsplitter 3055. A laser diode controller 3061 receives inputs A, B,C, and D from detectors 3011, 3013, 3015 and 3017, respectively. Laserdiode controller 3061 connectedly controls laser diode 3051.

In the most likely scenario the diode 3051 will be GaAs emitting lightin th 0.8 micron wavelength region, and the fiber multiplexer 3055 willbe designed to cross couple light at this wavelength. The fiber 3055will be doped with trace materials to support amplification at eitherthe 1.3 or 1.5 micron region, specifically the fiber amplificationsection would be designed to absorb strongly photons in the 0.8 micronwavelength region forming excited states that reemit photons in thelonger 1.3 to 1.5 micron region. The multiplexing element 3055 will bearranged so that light in this region passes directly through it. Thetaps 3001 and 3003, detectors 3011, 3013, 3015 and 3017 and laser diodecontroller 3061 operate in a manner analogous to the fiber opticrepeater of FIG. 29.

These above repeaters may be made in all optical format so that the datais not regenerated electronically at any point in the repeater box. Thisoffers the advantage of extremely long secure links without the need ofguarded repeater sations.

This approach could be used to support very long haul secure fiber opiclinks as required. Examples would include transcontinental andtransoceanic links as well as many shorter but significantly long linkson the order of hundreds of kilometers.

This is believed to be the first practical configuration of a coherentcommunication system with all optical repeaters. It is almost certainlythe first secure implementation of optical repeaters in a fiber opticcommunication system.

FIG. 31 shows an all optical repeater similar to that illustrated byFIG. 30. As before, taps 3101 and 3103 are found along fiber 3105 toconnect with detectors 3111, 3113, 3115 and 3117 having outputs A, B, C,and D, respectively. Inputs A, B. C, and D are made available to asemiconductor controller 3161. Semiconductor controller 3161 controls anoptically active semiconductor 3175 which is pigtailed into fiber 3105.The optically active semiconductor 3175 devices are currently beingbuilt and marketed by BTD a joint venture of British Telecom and Dupont.

The foregoing disclosure and description of the secure communicationsystem of the present invention is illustrative and explanatory thereof,and various changes in the specific transducers necessary to effectaction in the system, in the order of placement of the componentsdescribed herein, the system configuration, system scale, types ofmaterials and plan orientation, as well as in the details of theillustrated configuration shown herein, may be made without departingfrom the spirit and scope of the invention.

What is claimed is:
 1. A secure communication system comprising:a light source for producing light; a central beam splitter, having a first port optically connected to said light source, and a second, third, and fourth port, said beam splitter being adapted to receive a first beam of light from said light source, split said first beam of light into a second and a third beam of light, and recombine said second and said third beams into a fourth beam of light which is adapted to be received by said fourth port; a detector, connected to said second port of said central beam splitter, for detecting momentary and steady state phase shifts of said second and said third beams of light from said fourth light beam; a fiber optic path having a first end optically connected to said third port of said central beam splitter, a second end optically connected to said fourth port of said central beam splitter; and a midpoint located on said fiber optic path substantially equidistant from each of said first and second ends, for facilitating the propagation of said second and said third beams of light in counterpropagating directions; a phase modulating device, located along and optically connected to said fiber optic path and offset from the midpoint of said fiber optic path, for modulating light travelling toward and away from said phase modulating device; an alarm beam splitter, having a first and a second output port located along and optically connected to said fiber optic path, for coupling a portion of the light propagating within said fiber optic path from said fourth port of said central beam splitter into said second output port; a first light intensity detector, having an optical input optically connected to said first output port of said alarm beam splitter and an electrical output for detecting and generating an output electrical signal indicative of light intensity fluctuations propagating along said fiber optic path from said fourth port of said central beam splitter; and a second light intensity detector, having an optical input optically connected to said second output port of said alarm beam splitter, and an electrical output, for detecting and generating an output electrical signal indicative of light intensity fluctuations propagating along said fiber optic path from said third port of said central beam splitter.
 2. The secure communication system of claim 1 and further comprising a ratio transducer having a first output connected to said electrical output of said first light intensity detector, a second input connected to said second light intensity detector, and a ratio signal output, for comparing and outputting an electrical signal through said ratio signal output indicative of the electrical output signal from said first light intensity detector with the electrical output from said second light intensity detector.
 3. A secure communication system comprising:a light source for producing light; a central beam splitter, having a first port optically connected to said light source, and a second, third, and fourth port, said beam splitter being adapted to receive a first beam of light from said light source, split said first beam of light into a second and a third beam of light, and recombine said second and said third beams into a fourth beam of light which is adapted to be received by said fourth port; a detector, connected to said second port of said central beam splitter, for detecting momentary and steady state phase shifts of said second and said third beams of light from said fourth light beam; a fiber optic path having a first end optically connected to said third port of said central beam splitter, a second end optically connected to said fourth port of said central beam splitter, and a midpoint located on said fiber optic path substantially equidistant from each of said first and second ends, for facilitating the propagation of said second and said third beams of light in counterpropagating directions; a phase modulating device, located along and optically connected to said fiber optic path and offset from the midpoint of said fiber optic path, for modulating light travelling toward and away from said phase modulating device; a first alarm beam splitter located along and optically connected to said fiber optic path and having an optical output for coupling a portion of the light propagating within said fiber optic path from said fourth port of said central beam splitter into said optical output; a second alarm beam splitter located along and connected to said fiber optic path and having an optical output for coupling a portion of the light propagating within said fiber optic path from said third port of said central beam splitter into said optical output; a first light intensity detector, having an optical input optically connected to said output port of said first alarm beam splitter and an electrical output for detecting and generating an output electrical signal indicative of light intensity fluctuations propagating along said fiber optic path from said fourth port of said central beam splitter; and a second light intensity detector, having an optical input optically connected to said output port of said second alarm beam splitter, and an electrical output, for detecting and generating an output electrical signal indicative of light intensity fluctuations propagating along said fiber optic path from said third port of said central beam splitter.
 4. The secure communication system of claim 3 and further comprising a ratio transducer having a first output connected to said electrical output of said first light intensity detector, a second input connected to said second light intensity detector, and a ratio signal output, for comparing and outputting an electrical signal through said ratio signal output indicative of the electrical output signal from said first light intensity detector with the electrical output from said second light intensity detector.
 5. A secure communication transceiver comprising:a light source for producing a first beam of light having a first wavelength characteristic; a central beam splitter, having a first port optically connected to said light source, and a second, third and fourth port, said beam splitter being adapted to receive said first beam of light from said light source, split said first beam of light into a second and a third beam of light, and recombine said second and said third beams into a fourth beam of light which is adapted to be received by said second port; a detector, optically connected to said second port of said central beam splitter, for detecting momentary and steady state phase shifts of said second and said third beams of light from said fourth light beam; a first wavelength division multiplexing device having a first port optically connected to said fourth port of said central beam splitter, a second and a third port, for routing light having said first wavelength characteristic from said first port to said third port and from said third port to said first port, and for routing light having a second characteristic wavelength from said third port to said second port and from said second port to said third port; a second wavelength division multiplexing device having a first port optically connected to said third port of said central beam splitter, a second and a third port, for routing light having said first wavelength characteristic from said first port to said third port and from said third port to said first port, and for routing light having a second characteristic wavelength from said third port to said second port and from said second port to said third port; a first alarm beam splitter, having a first port optically connected to said second port of said first wavelength division multiplexing device, a second port and a third port, for routing light from said first port to said third port and said third port to said first port and for splitting a portion of the light routed from said first aport to said third port through said second port; a second alarm beam splitter, having a first port optically connected to said second port of said second wavelength division multiplexing device, a second port and a third port, for routing light from said first port to said third port and said third port to said first port and for splitting a portion of the light routed from said first aport to said third port through said second port; a light intensity detector, optically connected to said second ports of said first and second alarm beam splitters, for detecting light intensity fluctuations at said second ports of said first and second alarm beam splitters; and a phase modulating device, having a first port connected to said second port of said first alarm beam splitter and a second port connected to said second port of said second alarm beam splitter, for phase modulating light having said second wavelength characteristic passing in both directions through the first and second ports of the phase modulating device.
 6. A secure communication system, comprising a first and a second secure communication transceiver as claimed in claim 5 and wherein said third ports of said first and second wavelength division multiplexing devices of said first transceiver is optically connected to said third ports of said first and second wavelength division multiplexing devices, respectively, of said second transceiver, and wherein the first beam of light having a first wavelength characteristic of said first transceiver is equal to the second wavelength characteristic of the second transceiver, and the second wavelength characteristic of said first transceiver is equal to the first wavelength characteristic of the first transceiver, and said phase modulation device of said first transceiver is displaced from the center of a fiber optic path formed with respect to said central beam splitter of said second transceiver and wherein said phase modulation device of said second transceiver is displaced from the center of a fiber optic path formed with respect to said central beam splitter of said first transceiver.
 7. A communication system comprising:a light source for producing a first beam of light; a central beam splitter, having a first port optically connected to said light source, and a second, third, and fourth port, said beam splitter being adapted to receive a first beam of light from said light source, split said light into a second and a third beam of light, and recombine said second and said third beams into a fourth beam of light which is adapted to be received by said fourth port; a detector, connected to said second port of said central beam splitter, for detecting momentary and steady state phase shifts of said second and said third beams of light from said fourth light beam; a fiber optic path having a first end optically connected to said third port of said central beam splitter and a second end optically connected to said fourth port of said central beam splitter for facilitating the propagation of said second and said third beams of light in counterpropagating directions; a phase modulating device, located along said fiber optic path and in optical connection with said third and said fourth ports of said central beam splitter, for phase modulating said third and said fourth counterpropagating beams of light; doped fiber means, located at some point along said fiber optic path, for adapting said second and said third beams of light to an optical pumping function; and diode array means, adjacent to the length of said doped fiber means, for light pumping and increasing the intensity of said second and third beams of light within said doped fiber means, said doped fiber means and said diode array means forming a repeater station.
 8. The communication system of claim 7, and further comprising a pseudorandom pathlength generator, optically connected in series along said fiber optic path means, for pseudorandomly changing the optical pathlength between said third and said fourth ports of said central beam splitter.
 9. The communication system of claim 7 wherein said diode array means further comprises:a first diode array adjacent said doped fiber means; a second diode array adjacent said doped fiber means and on the opposite side of said doped fiber means with respect to said first diode array.
 10. A communication system, comprising:a light source for producing a first beam of light; a beam splitter, having first, second, third and fourth ports, said first port being optically connected to said light source, said beam splitter being adapted to receive said first beam of light into said first port, split said first beam of light into a second and a third beam of light through said fourth and third ports, respectively, and recombine said second and said third beams into a fourth beam of light into said second port; a detector, optically connected to said second port of said beam splitter, for detecting light received from said beam splitter; and a phase modulator, having a first port optically connected to said third port of said beam splitter and having a second port connected to said fourth port of said beam splitter, for modulating time varying signals upon light propagating toward and away from said beam splitter, said light source, beam splitter, and said detector being colocated to function as a receiver and said phase modulator being adapted to function as a transmitter.
 11. The communication system of claim 10 wherein said time varying signal is a sawtoothed wave.
 12. A secure fiber optic communication system comprising:a light source for inputting a beam of light into a first optical fiber; a first beam splitter having a first port optically connected to the first optical fiber, a second port optically connected to a first optical detector, and third and fourth ports optically connected to opposite ends of an optical fiber loop such that the third port receives a clockwise light beam from the loop propagating in a clockwise direction and the fourth port receives a counterclockwise light beam from the loop propagating in a counterclockwise direction; a signal source optically connected to the loop for modulating signals onto the light beams in the loop; an alarm means optically connected to the loop for detecting a change in the optical characteristics in the loop; and an electrical circuit optically connected to the first optical detector for demodulating the signal from the optical output received by the first detector from the light beams in the loop.
 13. The system of claim 12 wherein the signal source comprises a phase modulator.
 14. The system of claim 13 wherein said optical fiber loop includes a midpoint located substantially equidistant from each of said opposite ends, said phase modulator being offset from the midpoint of the loop.
 15. The system of claim 12 wherein a pseudorandom pathlength generator is optically connected to the loop.
 16. The system of claim 12 wherein the alarm means is an optical power measuring system comprising a second beam splitter optically connected to the optical fiber loop, one port of the second beam splitter being optically connected to a first optical power alarm detector for sensing the clockwise light beam and a second port being optically connected to a second optical power alarm detector for sensing the counterclockwise light beam, the alarm means further comprising a ratio transducer optically connected between outputs of the first and second alarm detectors to provide an alarm indication if an output power ratio from the first and second alarm detectors varies beyond a predetermined level.
 17. The system of claim 16 wherein the alarm means is an optical frequency measuring system comprising a plurality of said power measuring systems, each of said power measuring systems being optically connected to the loop wherein the second beam splitter of each passes to its corresponding first and second power alarm detectors only a single bandwidth of optical frequencies centered about one of a plurality of different optical frequencies contained within a predetermined spectrum transmitted from the light source means.
 18. The system of claim 12 wherein the alarm means is an optical power measuring system comprising a second beam splitter optically connected to the loop and having an output port which provides a portion of the clockwise light beam to a first optical power alarm detector, a third beam splitter optically connected to the loop and having an output port which provides a portion of the counterclockwise beam to a second optical power alarm detector, and a ratio transducer optically connected to outputs of the first and second power alarm detectors to provide an alarm indication if the output power ratio between the first and second power alarm detectors varies beyond a predetermined level. 